A III-V compound semiconductor crystal such as GaAs has a polarity (i.e., a direction of crystal) in its crystal structure. FIG. 5 is a perspective view showing the major crystal lattice planes of a GaAs crystal. The GaAs crystal structure belongs to the cubic system. Assuming that the top plane in FIG. 5 is a (100) plane, eight crystal lattice planes that are equivalent of a (111) plane in the simple cubic lattice (these crystal lattice planes are marked by cross hatching in FIG. 5 and they will be expressed by Ga{111} planes) are classified to a Ga plane on which only Ga atoms exist and an As plane on which only As atoms exist. In a direction perpendicular to such a crystal lattice plane, there appear alternately the Ga planes and the As planes with different distances having a three-to-one ratio. A viewing direction from the Ga plane to the nearest As plane is opposite to a viewing direction from the As plane to the nearest Ga plane, and it is considered that physical and chemical properties depend on such a direction. Such a difference in direction of crystal is referred to as a polarity. Among the eight crystal lattice planes belonging to Ga{111 }, the four equivalent crystal lattice planes (111), (1-1-1), (−11-1) and (−1-11) belonging to the Ga plane, these four crystal lattice planes being defined as a “positive polar plane”. On the other hand, the four equivalent crystal lattice planes (−1-1-1), (11-1), (1-11) and (−111) belong to the As plane, these four crystal lattice planes being defined as a “negative polar plane”. It is noted that the numbers in parentheses are the Miller indices and the minus sign should be attached to the trailing number after the minus sign. When the positive polar plane exists on the crystal surface, Ga atoms appear on the crystal surface. On the contrary, when the negative polar plane exists on the crystal surface, arsenic (As) atoms appear on the crystal surface.
The above-mentioned difference in polarity can not be recognized with the measurement using the ordinary X-ray diffraction method. Incidentally, a non-patent literature 1 described below discloses the polarity judgment with the use of a special X-ray diffraction method.
Non-patent literature 1: R. L. Barns and other two authors, “X-ray Determination of Polarity Sense by Anomalous Scattering at an Absorption Edge”, J. Appl. Cryst. (1970) 3, 27, p. 27-32
In the non-patent literature 1, there was measured the wavelength dependence of X-ray diffraction intensity of a GaAs crystal around the K absorption edge of Ga or As. The intensity of diffracted X-rays shorter in wavelength than the absorption edge was examined, and it was found that the X-ray diffraction intensity differs between the positive and negative polar planes, and thus the polarity of the GaAs crystal can be judged on the basis of the difference. Measurement of the wavelength dependence of X-ray diffraction intensity requires a variation on of the X-ray wavelength that will be incident of the sample. For meeting the requirement, there was used, in the non-patent literature 1, a combination on of a continuous wavelength range, which is generated by an X-ray tube having a heavy metal target, and a single crystal spectrometer.